p53 is known as an important factor for inhibiting canceration of cells. p53 is a transcription factor that induces the expression of genes involved in the cell cycle and cellular apoptosis in response to various stresses. p53 is thought to inhibit canceration of cells by a transcription regulating function thereof. In fact, deletion or mutation of the p53 gene is observed in about half of human cancer cases.
Meanwhile, overexpression of murine double minute 2 (Mdm2), a type of E3 ubiquitin ligase, is known as a factor for canceration of cells that are cancerated in spite of the presence of normal p53. Mdm2 is a protein of which expression is induced by p53. Mdm2 negatively regulates p53 by mediating degradation of p53 by binding to the transcription activity domain of p53 to decrease the transcription activity of p53, exporting p53 out of the nucleus, and further acting as a ubiquitination ligase against p53. Therefore, it is thought that inactivation of functions of and degradation of p53 are promoted in cells in which Mdm2 is overexpressed, resulting in canceration (Non Patent Document 1).
Paying attention to such functions of Mdm2, many approaches have been attempted using substances that inhibit the suppression of p53 functions by Mdm2, as candidate anti-tumor agents. Examples of the Mdm2 inhibitors targeting the Mdm2-p53 binding site have been reported, which include imidazoline derivatives having two sites substituted with halogenobenzene (for example, refer to Non Patent Documents 1 and 2 and Patent Documents 1 to 8) and imidazothiazole derivatives having two sites substituted with halogenobenzene (for example, refer to Patent Document 9). However, no report has demonstrated that these compounds actually showed efficacy in clinical practice.